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LEADING PROTON LEADING PROTON PRODUCTION in DIS at PRODUCTION in DIS at
HERAHERA
Lorenzo RinaldiLorenzo Rinaldi
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IntroductionIntroduction Leading protons (LP) carry large fraction of proton beam-energy (xL) and have small transverse momentum (pT)
LP production mainly soft process, non perturbative-QCD
Alternative approach needed (example: Reggeon-exchange)
Possible implication on LHC (simulation of forward protons) and extended cosmic ray showers
Measurement of LP production in DIS (Q2>3 GeV2, 45<W<225 GeV), for leading proton with 0.5<xL<1 (diffractive and not-diffractive regimes) and pT
2<0.5 GeV2:
LP longitudinal momentum spectrum
LP transverse momentum spectrum and slopes
LP rate vs DIS variables x,Q2
LP-tagged structure function
Comparison to models and other data
Results discussed in this talk:Results discussed in this talk:
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Leading proton production in ep collisionsLeading proton production in ep collisions
Standard fragmentationStandard fragmentation • LP from hadronization of p remnant• Implemented in MC models (Cluster, Lund strings...)
Virtual particle exchangeVirtual particle exchange
, IR, IPLP also from p fragmentation in double dissociative diffraction
,IR,IPp
LP cross sections vs structure functions:LP cross sections vs structure functions:(QCD-based approach)
22)4(2
2
4
2
22
224
,,,2
14,,,
TLLP
TL
TL pxQxFy
yxQdpdxdxdQ
pxQxd
KinematicsKinematicsLepton vertex variables: Lepton vertex variables: x, Q2, (W, y)LP variables: LP variables: pT
2,xL=ELP/Ep
LP and DIS variables assumed to be independent (vertex factorization)
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The ZEUS Leading Proton Spectrometer (LPS)The ZEUS Leading Proton Spectrometer (LPS)
6 stations (s1s6) each made by 6 Silicon-detector planes Stations positioned at 20 90m from I.P. Stations inserted at 10beam from the proton beam during data taking
xL < 1% pT
2 ~ few MeV2 (better than p-beam spread ~ 50-100 MeV)
Full LPS set used in this analysis (xL>0.5, possibility to extend xL>0.32)
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Cross section vs xCross section vs xLL
Cross section normalised to Cross section normalised to inclusive DIS cross sectioninclusive DIS cross section
Flat below diffractive peakFlat below diffractive peak
DIS and photoproduction DIS and photoproduction data are compatibledata are compatible
pT2<0.5 GeV2
pT2<0.04 GeV2
Non diffractive region
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Cross section vs pCross section vs pTT22 in bins of x in bins of xLL and slopes and slopes
Exponential behaviourExponential behaviour
Fit to A*exp(-b*pFit to A*exp(-b*pTT22) shown ) shown
with stat. errorwith stat. error
No strong dependence of No strong dependence of b on xb on xLL
-2GeV 7.6b
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),(
),,(),,(
22
2)3(22)3(
QxF
xQxFxQxr L
LP
LLP
Structure function ratio
Information on LP productionas a function of DIS variables
Measurement for 0.5<xL<0.92(NO diffraction)
Rates to inclusive DISRates to inclusive DIS
rrLP(3)LP(3) almost independent almost independent of x and Qof x and Q2 2 with average with average value value ~~0.40.4
Q2
x
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~~17% of DIS events have a LP 17% of DIS events have a LP with 0.5<xwith 0.5<xLL<0.92, almost <0.92, almost
independently of x and Qindependently of x and Q22
No clear evidence of vertex No clear evidence of vertex factorization violationfactorization violation
Rates to inclusive DIS - 2Rates to inclusive DIS - 2
),(
),(2
2
2)2(2)2(
QxF
QxFr
LPLP
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LP-tagged structure functionsLP-tagged structure functions
F2LP=rLP*F2
(ZEUS-S parametrization used)FF22
LPLP: same dependence on x and : same dependence on x and
QQ22 as F as F22
Q2
x
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Leading proton vs leading neutron dataLeading proton vs leading neutron data
Very similar behaviour xVery similar behaviour xLL<0.85<0.85
LP cross section almost twice LNLP cross section almost twice LN
In particle exchange model:In particle exchange model:expected from isovector: LP=1/2LNexpected from isovector: LP=1/2LNOther exchanges needed (isoscalars)Other exchanges needed (isoscalars)
Slopes comparable 0.7<xSlopes comparable 0.7<xLL<0.8 <0.8
where where exchange dominates exchange dominates
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Comparisons to Reggeon exchange modelComparisons to Reggeon exchange model
Very good decription in shapeVery good decription in shape
xxLL slighty underestimated slighty underestimated
b-slope slightly overestimatedb-slope slightly overestimated
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Summary and conclusionsSummary and conclusions
Cross section vs xL flat below the diffractive peak
LP cross section vs pT2 falls exponentially with a mean
slope ~6.7 GeV-2, approximately independent on xL
the rates rLP(2) and rLP(3) show no dependence on x and Q2 and LP structure functions proportional to F2
No evidence of vertex factorization violation
LP and LN: agreement with expectations from particle-exchange models
A Regge-inspired model reproduces quite well the LP features
LP production measured in DIS:LP production measured in DIS: